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I know that is roundabout way ask a question about perpetual motion but if I mentioned that in the title. No one would read it 😁 I guess I really need some help with my paradoxical thought. So here goes. it occured to me that one could setup a perpetual motion machine that could perform meaningful work if you had a permanent magnet, a superconductor and some feromagnetic material. There is an old magnetic perpetual motion idea enter image description here Now anyone who has played with magnets enough will realise this is impossible because the iron ball will just jump over the hole stick to the magnet. There is no way the ball will fall through the hole. But it struck me while watching a doco about superconductors that. If you could block the magnetic field with a semiconductor, you could make this silly design work. Now I know you cant. But I want to understand why. And I suspect it's not because room temperature superconductors break fundamental laws of physics.

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    $\begingroup$ There is nothing special about room temperature. If a room-temperature superconductor would break the laws of thermal dynamics, a liquid-nitrogen one would as well. (And I know this doesn't answer your real question, which is why it's a comment.) $\endgroup$ Feb 18 '20 at 13:37
  • $\begingroup$ But do nitrogen cooled superconductors really have zero electrical resistance or just approach it? $\endgroup$
    – Justin
    Feb 18 '20 at 19:40
  • $\begingroup$ They approach it closely enough that this isn't the reason your perpetual motion idea fails. $\endgroup$ Feb 18 '20 at 21:17